The present invention relates to a cycloconverter for supplying an electric power of a variable frequency to an AC motor such as an induction motor or the like. More particularly, the invention is concerned with a method of controlling a circulating-current type cycloconverter in which a circulatory current circulates over and through a positive converter and a negative converter. The invention is also directed to an apparatus for controlling the circulating-current type cycloconverter.
As is known in the art, the cycloconverter is an apparatus for linking together AC input voltage waveforms to thereby produce an AC output voltage having a frequency which differs from that of the AC input voltage. Since the circulating-current type cycloconverter among others is capable of changing over in a continuous manner the conduction periods of thyristors provided at positive and negative banks, respectively, the output current waveform of this type cycloconverter can approximate a sinusoidal waveform, whereby unwanted torque ripple tending to occur at a low rotational speed of the motor constituting a load of the converter can be suppressed, to advantage. On the other hand, the circulating-current type cycloconverter suffers a disadvantage in that the source power factor is lowered because the whole circulating current constitutes a lagging reactive component.
As a hitherto known apparatus for compensating for the reactive component of the output power of the circulating-current type cycloconverter, there may be mentioned the one described in Japanese patent publication JP-B-53552. The reactive power control system disclosed in this publication is implemented in such an arrangement in which the circulating-currents of individual phases in a cycloconverter are so proportioned that the circulating current is reduced in the phase in which the load current has a great absolute value while the circulating current is increased in the phase of the load current having a smaller absolute value, to thereby control the reactive power through coordination with leading reactive components produced by phase advancing capacitors provided at the power receiving end without need for increasing significantly the capacity of the cycloconverter.
However, in the prior known reactive power control system, no consideration is paid to the control of the circulating current in the overload condition. Consequently, upon occurrence of overload condition, voltage drop appearing across a direct current (DC) reactor is increased, whereby insufficiency in the output voltage of the cycloconverter is involved to give rise to a problem that desired torque can not be obtained from the AC motor constituting the load. For solving this problem, it may occur to increase the source voltage of the cycloconverter to the level of the voltage required by the load in the overload condition (which voltage is equal to a sum of voltage drop across the DC reactor and the voltage supplied to the load). With such measures, however, the reactive component is increased in the light-load condition, causing the source power factor to be lowered undesirably. For absorbing the increased portion of the reactive component by means of the phase advancing capacitor provided at the power receiving end, it is necessary to increase the capacity of the phase advancing capacitor.